This is the moment when the world's largest predator came face-to-face with two small children padding in the same pool. Instead of imminent danger there was mild curiosity on both sides.  For all is not as it seems. The interested children were separated from the polar bears - among the animal world's finest swimmers - looming over them by 10in of thick Plexiglass.  The coming together of children and beast was part of a unique initiative by the owners of the Cochrane Polar Bear Habitat in Canada, which only takes in bears rescued from the wild.  It allows visitors to get close enough to the creatures to try to promote understanding of their plight.  The polar bear's existence is in danger from the shrinking arctic sea ice. They swim from land to ice floes as they hunts seals - but global warming is forcing them to cover ever-larger distances.